Geology of part of the eastern margin of the Tanzanian Craton in the Mpwapwa area and its relationships with an off-craton, high-grade supracrustal gneiss sequence (Mpwapwa Group) of possible Palaeoproterozoic age

Abstract/Summary

A number of traverses have been undertaken across a ca. 45 km section of the north-south oriented eastern margin of the Tanzanian Craton between Dodoma and Mpwapwa, Central Tanzania. The boundary is a SE-dipping zone of high strain between about 1 and 2 km wide. The rocks of the eastern craton are uniformly composed of coarse-grained grey granodioritic, migmatised orthogneisses which are heterogeneous at outcrop scale, but are regionally homogeneous. The orthogneisses have no regionally consistent fabric and foliations are variably oriented at outcrop scale. However, there is a gradual increase in strain eastwards towards the edge of the craton, manifest as an increasingly strong, regionally consistent, SE-dipping foliation. This strain increase eventually leads to mylonitic and porphyroclastic planar fabrics and strong, uniformly SE-plunging, linear fabrics. The kinematics of the high-strain mylonites show a consistent top-to-the-NW sense of movement. The frontal thrust zone grades laterally into steep sinistral and dextral oblique strike-slip shear zones to the north and south respectively. The contact is a single wide thrust zone in the north and south section of the studied area, with an imbricate belt in the central part.
To the east, the cratonic rocks are in contact with a high-grade supracrustal succession, termed here the Mpwapwa Group in the light of uncertain regional correlations (= “Isimani Suite”?). It consists of a thick sequence of leucocratic quartzo-feldspathic gneisses and migmatites, semi-pelitic two mica-kyanite-garnet schists/gneisses, quartzites, marbles and calc-silicate rocks and abundant metabasic layers. There appears to be an east-west zonation of Mpwapwa Group lithological units, with most of the quartzites, calcareous rocks and pelitic schists/gneisses tending to occur close to the craton margin, with semi-pelitic gneiss/migmatite to the east, along with interlayered repetitions of bimodal acid, quartzo-feldspathic leucogneisses and mafic gneisses (amphibolite, mafic garnet amphibolite).
Mineral assemblages, as evidenced by garnet-kyanite in pelitic rocks, garnet-clinopyroxene in some metabasites suggest metamorphism under moderate to high pressure amphibolite facies, as might be expected at the base of a thrust stack and resulting crustal thickening. Possibly, therefore, the Mpwapwa Group was deposited in a rifted passive-margin setting at the edge of the Tanzanian Craton, with shallow marine environments at the immediate continental margin and bimodal volcanic rocks more distally. During collision orogeny, thrusting took place at this rifted margin, inverted the Mpwapwa Group basin and transported the supracrustal rocks over the craton margin, an event which telescoped, but did not obliterate the original depositional zonation. The group may thus be viewed as a parauthochthonous succession. The rocks were intruded by plutons of largely unfoliated biotite granite, two-pyroxene charnockite and tonalite, the ages of which are unknown. With the above hypothesis and uncertainties in mind, a suite of samples are undergoing U-Pb zircon dating in order to constrain the timing of these events.